On any federal-aid highway, the location, form, and character of informational, regulatory, and warning guideposts, direction boards, signs, curb, and pavement or other markings, and traffic signals, installed or placed by or under the authority of any county, shall be subject to the approval of the director of transportation or the director’s authorized representative. The director shall exercise the authority conferred upon the director in such manner as to protect and promote the securing of federal-aid for highways.